Integrated solar cells comprising amorphous silicon (hereinafter abbreviated as a-Si) having a p-i-n junction typically have the layer structure shown in FIG. 1. The integrated solar cells of this type are prepared as follows.
Transparent electrode layer 2 comprising a transparent conductive film of tin oxide (SnO.sub.2), etc. is formed on glass substrate 1 to a thickness of about 0.5 to 1.0 .mu.m by thermal CVD. On transparent electrode layer 2 are deposited a-Si layer 3 (e.g., p-type a-Si layer, hereinafter referred to as player) having a thickness of about 200 .ANG., intrinsic a-Si layer 4 (hereinafter referred to as i layer) having a thickness of about 0.1 .mu.m, and n-type microcrystalline silicon layer 5 (hereinafter referred to as n(.mu.c-si) layer) having a thickness of about 200 .ANG. in this order by plasma discharge decomposition to form a first cell. Gases to be used for p layer formation are silane (SiH.sub.4), diborane (B.sub.2 H.sub.6), and methane (CH.sub.4), and gases to be used for n(.mu.c-Si) layer formation are SiH.sub.4, hydrogen (H.sub.2), and phosphine (PH.sub.3).
On n(.mu.c-Si) layer 5 of the first cell are further deposited p layer 6, i layer 7 and n(.mu.c-Si) layer 8 in this order to form a second cell having a p-i-n junction. The film forming conditions for the second cell are virtually the same as for the first cell, except that the i layer of the second cell is as thick as 0.4 .mu.m. Finally, back electrode 9 is formed out of a metal, such as aluminum or silver, to a thickness of at least 2000 .ANG. by sputtering or vacuum evaporation. A transparent conductive film or a conductive paste may also serve as a back electrode.
In an attempt to increase conversion efficiency by integrated unit cells, it is necessary to make the n-p junction having a better ohmic contact between a first cell and a second cell. To this effect, conventional solar cells have used an n(.mu.c-Si) layer as an n-type layer of the first cell, with its thickness reduced to about 100 to 400 .ANG. so as to minimize the loss due to light absorption by the layer itself.
In general, however, reduction of the thickness of a microcrystalline film results in failure of microcrystallization. It follows that electrical conductivity drastically decreases, making it difficult to maintain ohmic properties.